I. Field of the Invention
The present invention is related generally to electrochemical cells and to devices and methods for electrochemically determining the concentrations of one or more desired species of interest in a sample solution. More particularly, the invention is directed to a new planar, free diffusion reference electrode or reference half cell for use in combination with one or more additional electrodes making up a sample analyzing or sensor half cell used to make quantitative concentration determinations.
II. Related Art
Methods and devices utilized for determining concentration of electroactive species in solution using electrochemical or electrolytic methods such as, for example, the determination of pH, pCO.sub.2 and electrolytes in blood samples, are well known. These instruments typically include a pair of electrochemical half cells, one of which is used as the sensor or sample analyzing half cell and the other as a reference electrode or reference half cell. The sensor half cell is normally provided with a membrane which forms complexes with the specific ion or ions to be measured in the sample. A voltage or potential is developed across the membrane that is proportional to the concentration of the species of interest in the sample. Generally, in accordance with the Nernst equation, by knowing the ion concentration of the species of interest on the internal side of the sensor membrane and measuring the potential across the membrane, it is possible to readily determine the unknown ion concentration on the opposite (sample) side of the membrane.
Theoretically, to make an accurate measurement, the reference half cell must maintain a constant potential. It is typically a metal/metal salt couple in equilibrium with the anions of the salt materials such as silver/silver chloride (Ag/AgCl) in equilibrium with chloride ions (Cl.sup.-). The stability of the reference half cell potential depends on the stability of the anion concentration in the reference half cell electrolyte medium. It is used with the sample measurement electrode to complete the electrical circuit through the sample.
The reference electrode or half cell is generally constructed similar to the sensor half cell, except that, whereas the membrane separating the electrode itself from the sample solution in the case of the sensor half cell is normally rather species-specific with respect to transport across the membrane, the material of the reference half cell is normally not specific to a particular species. Such materials as glass frits and various porous ceramic separator materials, or even small open weep holes, typically isolate the reference electrolyte medium from that of the sample or sensor half cell. Separation is such that mixing of the two is minimized. An additional technique involves an ion-conductive salt bridge or bridge electrolyte as the conduction mechanism to complete the electrical circuit between the reference half cell and the sensor or sample half cell.
Reference electrodes as typically constructed are relatively large and expensive, requiring many distinct components which are precision assembled. Such a reference electrode half cell is illustrated and described, for example, by Dohner et al in "Reference Electrode with Free-Flowing Free-Diffusion Liquid Junction", Anal. Chem., 58:2585-2589 (1986).
Present reference electrodes as typically constructed also contain large volumes (0.5-10 ml) of an internal electrolyte solution. The internal electrolyte and bridge solution defines the equilibrium potential of the electrode, and the junction potential created with introduction of the sample. In addition to being rather complicated and expensive to produce, the large number of pieces involved and the large volume of internal electrolyte solution make any change of the internal and bridge electrolyte a very difficult and time consuming task.
One significant problem with glass frit and other porous separator materials involves the clogging of the junction by high molecular weight contaminants. A clogged junction typically renders the reference electrode unsuitable or unusable because of drift or high impedance. This problem is particularly acute when the reference electrode is used in a device making measurements in biological solutions such as blood, plasma, or serum.
Another significant problem with glass frit and other porous separator materials is that junction potentials created are likely to produce misleading results (see Dohner et al, supra). Errors of 10 mV or higher can occur because junction potentials deviate from expected values.
Free diffusion type reference electrodes have been found to be free of such artifacts, but are typically very large and elaborate in construction, often requiring pumping mechanisms to maintain uni-directional flow of electrolyte. These systems are far too elaborate to be used with portable or disposable electrochemical measuring systems.
More recently, attempts have been made to solve some of these problems, particularly size reduction, by the use of a planar electrode structure such as disclosed by Lauks in U.S. Pat. No. 4,933,048. While the Lauks system successfully miniaturizes the electrode, it does suffer from several drawbacks. The Lauks device is designed to be stored in a dry non-conductive state until activated by moisture immediately prior to use. Lauks further teaches the use of a liquid permeable, ion impermeable layer to isolate the internal reference electrode from the sample environment. The liquid permeability of the layer facilitates rapid activation of the sensor by water transfer from a sample medium. The system, however, does require a wet-up period, which makes it unavailable for use during that time. Additionally, the Lauks liquid permeable configuration is subject to inaccuracies created by osmotic imbalances between reference internal and sample solutions. For example, when a sample having a lower solute concentration than that of the internal electrolyte layer is measured, osmotic pressure forces water to flow through the external membrane into the internal electrolyte layer, diluting chloride concentration (Lauks ion X) and causing reference instability. Osmotic errors might be significant when making measurements in unknown samples such as whole blood, where osmolarity of normal samples may vary by 10% or more.
Another planar reference electrode with a liquid impermeable external membrane, but complicated multi-layered system for partially exposing a salt bridge is shown by Pace in U.S. Pat. No. 4,454,007. Another planar multiple layered reference sensor has been described by Yamaguchi in U.S. Pat. No. 5,066,383.
These devices, while somewhat successful, are characterized by a very high bridge impedance which is undesirable. All these prior attempts at miniaturization of reference electrodes in a planar configuration suffer from inaccuracies associated with restrained liquid junctions.
Accordingly, it is a primary object of the present invention to provide a simple, miniaturized planar reference electrode with the accuracy of a free diffusion type junction.
Another object of the invention is to provide a reference electrode which can be stored in a conductive or ready-to-use state requiring no wet-up period.
Still another object of the present invention to provide a reference electrode having the flexibility that enables easy adjustment of internal fill solutions and bridge electrolytes by a simple procedure, in order to customize the reference potential for a particular application.
A further object of the present invention to provide an improved free diffusion junction between the reference half cell and the sensing half cell which substantially precludes undesirable ion migration between the two during an ample measurement period for a one-use, disposable electrochemical cell system.
Yet another object of the present invention to provide a new and improved, substantially solid state reference electrode including a low impedance junction between a reference electrolyte medium and a sample to be tested.